An appreciable portion of the oil production in the United States is from reservoirs associated with piercement salt domes in the Gulf of Mexico region and in the Paradox Basin. The general location of the salt dome is known from surface exploration, but the locations of its flanks, especially at certain depths where possible pay sands may be truncated by the salt, are not known with an accuracy that would be sufficient for oil field development.
The geometry of the salt flank and overhang associated with quite a few of these salt domes is not precisely known. With the refocusing of the U.S. oil industry in the direction of development drilling and a de-emphasis on exploration, there has been a resurgence in interest to define these producing salt dome geometries more exactly. This will ultimately allow a better quantification of any unrealized updip hydrocarbon potential associated with these producing fields.
In specific areas however, such as the East Texas Basin, the salt dome flanks are relatively unexplored. Seismic surveys refracting energy through the salt and then recording the wavefield in an adjacent well (known as salt proximity surveys), are of limited use in this area. The high sedimentary rock velocities encountered relatively shallow in the stratigraphic section preclude the success of the downhole refraction technique to aid in defining the salt flank geometry.
Since the 1930's, two types of borehole seismic surveys have been used to define the shape of salt domes--radial refraction surveys and proximity surveys (McCollum and LaRue, "Utilization of Existing Wells in Seismograph Work," Amer. Assn. of Petroleum Geologists Bull. 5, No. 12, pp. 1409-1417, 1931). In the radial refraction survey, (such as the surveys disclosed by L. W. Gardner in "Seismograph Determination of Salt-dome Boundary Using Well Detector Deep on Dome Flank," Geophysics, v. 14, pp. 29-38, 1949), a downhole three component (3C) phone is placed inside a salt dome usually at a depth greater than the objective depth. Typically, an exploration well that drilled on the flank of the dome and bottomed in or near salt is used as the receiver well. Shots are fired in a pattern resembling spokes radiating from the opposite flank of the dome. Since these surveys were carried out before the development of downhole gyroscopes, the geophone orientation was not known. Only travel time is provided as output, and the interpreter has to estimate the sediment velocities, to generate a 3-dimensional surface of possible solutions. Several seismic shots are needed so that these 3-D surfaces can be lined up. Using the direct arrival time and salt and sediment velocities, the surface of all solutions that fit the observed time is displayed in either vertical or horizontal slices. The tangent to these "aplanatic" surfaces provides the estimate of the salt-sediment interface.
However, conducting a radial refraction survey with wireline tools has severe drawbacks. The cost is excessive. A 20-level radial refraction survey with over 200 source points recorded into a single geophone, with offsets up to 20,000 feet and a wide azimuth range would not be economically feasible with prior art procedures. Shots would have to be repeated at each level, so 200 source points with 20 levels would require 4000 shots.
Proximity surveys are designed to image the salt flank very near the wellbore. In this method, a source is placed over the top of a dome with a downhole 3C geophone in sediments on the flank of the dome. Since the 1980's, these surveys have been recorded with a gyroscopically oriented 3C phone which reduces the range of possible solutions for the salt-sediment interface from an ambiguous 3D aplanat to a unique point in space. The use of gyroscopes to determine the orientation of geophones in a wellbore is disclosed in U.S. Pat. No. 4,800,981 issued to Uttecht et al. and by A. Manzur in "Delineation of Salt Masses Using Borehole Seismics" in the Oil and Gas Journal, Oct. 7, 1985.
A proximity survey only provides useful information pertaining to the location of a salt flank that is within a few hundred feet from the wellbore. Therefore, only a very narrow vertical slice of information along the well path is obtained.
The use of compressional, or P wave reflection data in geophysics analysis is well known. A typical seismic reflection prospecting system which produces compressional wave reflection data would be comprised of a compressional wave source located on the surface and geophones spaced along a line of exploration on the surface for measuring the vertical component of the ground motion caused by the reflected compressional wave. However, conventional P- waves travelling through the subsurface also produce vertically-polarized shear, or converted S- waves when reflected at other than a normal angle of incidence. Thus, seismic sections produced by such compressional wave seismic exploration systems would contain two types of wave information which, if properly exploited, will yield useful information regarding the lithologic characteristics of the subsurface formation under investigation. In recent years, interest has been growing in obtaining shear wave information to provide useful information regarding the lithological characteristics of the subsurface formation under investigation. Such information, if properly obtained and exploited, can be utilized in conjunction with information obtained from compressional wave seismic section to provide a more detailed analysis of the characteristics of the subsurface formation. For example, compressional wave seismic sections can provide useful information on the compressibility of subsurface formations, while shear wave seismic sections can provide useful information on subsurface formation rigidity.
Several limitations related to the characteristics of the shear wave have, however, prevented the full exploitation of shear wave information. Shear wave seismic reflections are noisier than compressional wave seismic reflections, making proper interpretation difficult. Furthermore, the direct propagation of a shear wave into a subsurface formation to induce a shear wave reflection requires special transducers and additional steps over and above those required for obtaining compressional wave reflection data. This makes obtaining shear wave reflection data difficult, more costly and time-consuming.
Three U.S. patents have issued to C. W. Frasier and have been assigned to Applicant's assignee. U.S. Pat. No. 4,611,311 discloses a method of collecting and stacking converted wave reflections to reinforce the reflections at the proper subsurface location (at common reflection points), using recording methods known in the art.
U.S. Pat. Nos. 4,597,066 and 4,596,005, also issued to C. W. Frasier, illustrates that for dipping geologic layers, the conventional (P-P) reflections also must be collected and stacked using a common reflection point method, as the dip angles cause the reflection paths to be non-symmetric.
U.S. Pat. No. 4,881,209, issued to Bloomquist et al., teaches a method of correcting converted P- to S- wave data for normal moveout, from standard reflection events. P- to S- wave velocity ratios are selected and collections of traces corresponding to a selected ratio are stacked and the series of stacks are correlated to the originally collected stacked data to determine the correct Vp/Vs ratio and shear wave velocity.
The patents issued to Frasier and Bloomquist all are concerned with reflection data in a medium having nearly constant velocity. As P- wave velocity is known and S- wave velocity is not known, certain approximations must be made.
There is therefore a need for a method of migrating transmitted seismic data that penetrates a geologic unit having an anomalous velocity that is independent of the velocities of the surrounding sediments.
The prior work is limited in the attempts at delineating the flanks of a salt mass in that no suitable method can economically and accurately delineate the salt mass flanks over large portions of the salt mass. A 3D surface seismic survey would provide adequate results, but would cost at least ten times as much. There is, therefore, a need for such a method for use in the geophysical exploration for oil and gas.